Zhuojin Pipe Fitting Co., LtdSubmerged Arc Surfacing of Inconel 625 on Flange Sealing Surfaces
Literature Overview
This technical paper by Li Pei from Xi'an University of Architecture and Technology addresses a specific and challenging engineering problem: the application of Inconel 625 overlay on the sealing surface of S31603 (equivalent to ASTM A351 CF8M / 316 cast stainless steel) Class III flanges. Published in "Chemical Equipment Technology" in 2021, the paper documents a complete welding procedure qualification process for this specialized surfacing application, reflecting the growing demand for corrosion-resistant sealing surfaces in aggressive chemical processing environments.
Technical Background and Challenges
Inconel 625 is a nickel-chromium-molybdenum superalloy (UNS N06625) with excellent resistance to pitting, crevice corrosion, and stress corrosion cracking in chlorinated environments. Its application on flange sealing surfaces addresses the common problem of gasket face corrosion in high-chloride service conditions. However, surfacing Inconel 625 on austenitic stainless steel presents several metallurgical challenges:
| Challenge | Technical Consideration |
|---|---|
| Dilution control | Nickel content in the overlay must remain high enough to maintain corrosion resistance |
| Cracking susceptibility | High nickel alloys are susceptible to hot cracking during solidification |
| Dilution from base metal | S31603 base metal dilution reduces overlay alloy content |
| Ferrite content | Excessive ferrite can impair corrosion resistance; insufficient ferrite increases cracking risk |
| Surface flatness | Flange sealing surfaces require precise flatness and finish |
Welding Procedure Qualification Methodology
The paper describes a systematic approach to procedure qualification following established standards:
Inspection Methods
| Inspection Method | Purpose |
|---|---|
| Macro examination | Verification of weld appearance, penetration, and coverage |
| Penetrant testing (PT) | Detection of surface and near-surface defects (cracks, pores) |
| Chemical composition analysis | Verification of alloy content and dilution level |
| Ferrite content determination | Control of microstructure to prevent cracking and ensure corrosion resistance |
Process Parameters
Submerged arc welding (SAW) for Inconel 625 surfacing typically employs the following parameter ranges:
- Current: 200-400 A (DCEN preferred for nickel-base alloys)
- Voltage: 25-35 V
- Travel speed: 150-300 mm/min
- Flux: Rutile-type or basic-type low-hydrogen flux
- Wire: ERNiCrMo-3 (AWS A5.11) or equivalent Inconel 625 filler
The use of DCEN polarity is important as it provides deeper penetration and better wetting for nickel-base alloys, while reducing the risk of tungsten inclusion (not applicable in SAW but relevant for arc stability).
Engineering Practice Integration
Flange Sealing Surface Requirements
For ASME B16.5 or EN 1092-1 flanges, the sealing surface (RF or RTJ) must maintain:
- Surface roughness: typically Ra 3.2-6.3 μm for raised face flanges
- Flatness: within 0.05 mm for standard ratings
- Hardness: generally not exceeding 250 HB for gasket compatibility
- Corrosion resistance: matching or exceeding the service environment requirements
Quality Control Protocol
The welding procedure qualification should follow a PDCA cycle:
- Plan: Define performance requirements, select consumables, establish parameters
- Do: Execute welding trials with the proposed procedure
- Check: Perform all required inspections (PT, chemistry, ferrite, macro)
- Act: Adjust parameters based on results and re-qualify if necessary
Critical Success Factors
Based on engineering experience, the following factors are critical for successful Inconel 625 surfacing on stainless steel flanges:
- Preheating to 150-250°C to reduce cooling rate and minimize cracking
- Interpass temperature control below 250°C to prevent excessive grain growth
- Minimum overlay thickness of 3-5 mm to ensure adequate dilution resistance
- Post-weld grinding to achieve required surface finish without exposing the base metal
Key Questions and Reflections
A significant consideration not fully addressed in the paper is the long-term performance of the Inconel 625 overlay under cyclic thermal and mechanical loading typical of flange connections. The coefficient of thermal expansion mismatch between Inconel 625 (12.8 × 10⁻⁶/°C) and S31603 (16.3 × 10⁻⁶/°C) can induce residual stresses that may compromise the overlay integrity over time. Additionally, the grinding process required to achieve surface finish specifications may introduce surface defects or residual stresses that could initiate cracking in the nickel-base overlay.
The ferrite content determination mentioned in the paper is particularly important. For Inconel 625, the target ferrite content should be below 5% to maintain corrosion resistance, while for the S31603 base metal, 5-30% ferrite is acceptable. The dilution at the fusion line creates a transition zone where ferrite content may vary significantly, potentially creating a localized susceptibility to stress corrosion cracking.
Study Insights and Implications
This paper provides a practical reference for engineers facing the challenge of upgrading existing flange sealing surfaces for more aggressive service conditions. The systematic approach to procedure qualification, combining multiple inspection methods, represents good engineering practice. The key takeaway is that successful surfacing of dissimilar alloys on precision-machined surfaces requires careful attention to dilution control, residual stress management, and surface finish maintenance. Engineers should always consider the full service life of the component, including thermal cycling, pressure cycling, and potential gasket-induced stress concentrations, when specifying surfacing procedures for critical sealing surfaces.